1. Field of the Invention
The present invention relates to a surface treated metal member having an excellent resistance against sliding wear, rolling wear, line wear, wear fatigue and the like, and its manufacturing method. The surface treated metal member is useful for sliding members for an automobile, motor-bicycle (two-wheeled vehicle), bicycle and the like such as a connecting rod, connecting rod pin, piston head, valve spring retainer, seat rail, inner sleeve, oil pump, valve lifter, crank shaft, and cylinder liner; spring members such as a valve spring; gear members such as a bicycle gear and motor-bicycle sprocket; shaft members contact with various bearings such as a bicycle rear pulley, bicycle pedal shaft, automobile crank shaft; and contact parts with power transmission members. Also, such a metal member is suitable for jig and tool members such as a screw and press die; and compressor members of scrolls. In addition, it is useful for non-power transmission members such as an automobile wheel, golf head, and plate member of a cooking hot plate, and further it is effectively used as a means for increasing wear resistance and corrosion resistance.
2. Description of the Related Art
Ti or a Ti alloy (sometimes, represented by a Ti alloy), Al or an Al alloy is a lightweight material having an excellent specific strength. These alloys, therefore, have been extensively used for structural members in the chemical industrial field, and in airplane and space transport field. In particular, as transport machines including automobiles have been highly graded, functions such as comfortable running and safety running have been required to be enhanced, and as a consequence the number of additional function portions have been further increased. This presents a problem in increasing a vehicular weight. On the other hand, it has been further required to reduce fuel consumption and exhaust gas by lightweightness of vehicular bodies. To solve these problems, the conventional steel materials have come to be replaced with Ti or Al alloys as lightweight metal materials.
These alloys, however, have a disadvantage in that it is poor in wear resistance and seizure resistance, and accordingly, for sliding members and shaft members of machines, an attempt for enhancing the wear resistance of these alloys have been made by applying, on the surface of, for example, the Ti alloy, wet-plating such as Ni--P plating or Cr plating, thermal diffusion such as ion nitriding and boronizing, overlaying or thermal spraying. In case of Al alloys, anodings or some platings have been applied in order to improve their wear resistance. However, in wet-plating such as Ni--P plating or Cr plating, the hardness and toughness of a plating layer are low, so that the wear resistance is insufficient. On the other hand, in case of Ti or Ti alloys, thermal diffusion such as ion nitriding and boronizing requires the treatment for a long time at a high temperature (about 1000.degree. C.), tending to coarsen crystal grains by the growth of crystal grains and hence to deteriorate mechanical properties. In thermal spraying, a large deformation is easily generated due to thermal strain; cracking is possibly generated in weld and bonding failure is sometimes generated; secondary machining such as grinding is required after welding; and fine members are difficult to be processed.
The methods of improving the wear resistance of, for example, a Ti alloy have many problems. Of these methods, wet-plating such as Ni--P plating or Cr plating has a possibility in relatively easily improving the wear resistance by taking a good balance between the hardness and toughness of the plating layer. In particular, to enhance the wear resistance of a Ti alloy, a method using Ni--P plating being excellent in toughness, lubricity and precipitating efficiency has been proposed. Specifically, in this method, Ni--P plating is combined with heat-treatment, and further with the subsequent blasting of fine particles such as shot peening or dry honing (hereinafter, sometimes referred to as "honing treatment"). For example, a technique of heat-treating a Ni--P plating layer has been disclosed in Unexamined Japanese Patent Publication No. HEI 2-221377. A technique of heat-treating a Ni--P plating layer, and then blasting fine particles to the surface of the plating layer by shot-peening or dry honing has been disclosed in Unexamined Japanese Patent Publication Nos. HEI 2-133578, SHO 63-312982 and HEI 1-159358. In addition, similar techniques have been also disclosed in Unexamined Japanese Patent Publication Nos. HEI 4-246181 and HEI 5-78859.
Incidentally, the heat-treatment adopted in the above-described techniques is intended to increase the hardness of a Ni--P plating layer or form a mutual diffusion layer at the interface between the plating layer and the base member, and hence to improve the adhesiveness therebetween; however, the disclosed heat-treatment condition in the above-described references is set to increase the hardness of a plating layer, and during this heat-treatment a large tensile strength is generated on the plating layer, thereby deteriorating the toughness. Moreover, the adhesiveness is slightly improved by the formation of the diffusion layer at the interface between the plating layer and the base member by heat-treatment; however, the amount of the diffusion layer is insufficient to significantly improve the wear resistance.
On the other hand, shot peening or dry honing performed after heat-treatment is intended to impart a residual compressive stress on the plating layer reduced in toughness by heat-treatment for recovering the toughness, and to enhance the fatigue strength of the base member using the hoop fastening effect of the plating layer. However, the residual compressive stress applied to the plating layer tends to be insufficient and thereby the toughness is difficult to be recovered, and further cracks are easily generated from the damaged portions formed on the surface upon shot peening, thus failing to sufficiently improve the wear resistance.